PRACTICAL ORGANIC CHEMISTRY
water running back when the aspirator is stopped. Before
stopping the pump, close the glass tap. Turn off the water, and
then lift the tap out of its socket for a moment to equalise the
Use a porcelain funnel and filter flask, different sizes of which
are shown in Fig. 36. The bottom of the funnel is covered with
a disc of filter paper. After filtering, wash three or four times
with a little cold water,
press the precipitate
well down and let it
drain. Remove the
precipitate and spread
it on a piece of porous
plate, and place it in
over sulphuric acid.
There are several use-
ful forms of vacuum-
desiccator, two of
which are represented
in Fig. 37.
The ground rims
are greased with vase-
line or a mixture of
bees-wax and vaseline,
and the air is exhausted
by attaching the tube
of the water-pump to
FIG. 35. the glass tap of the
If the substance is left overnight in the desiccator it will be
dry by the next day. The silver salt should be protected as far
as possible from the light. When the precipitate is thoroughly
dry, weigh about 0-3 gram into a weighed porcelain crucible.
Cover with the lid and heat, at first gently, over a small flame.
When the first reaction is over, heat the crucible for a few
minutes to a dull red heat, and then allow it to cool in a desic-
cator. The silver salt will be completely decomposed and leave
a dull white residue of silver. The crucible is now weighed and
the weight of silver determined.